The Castro boys now have a U.S. Hellfire missile to share with Kim Jong Un.

You’d think that North Korean dictator Kim Jong Un wouldn’t have a friend in the world these days. His relentless pursuit of weapons of mass destruction and willingness to starve his own people is evil madness. Last week even communist China condemned the supreme leader’s fourth nuclear test, which the chubby little psychopath called “the thrilling sound of our first hydrogen bomb explosion.”

But Mr. Kim is not all alone. He still has the Caribbean’s Cosa Nostra—aka the Castro family—as a friend and ally. The Cold War may be long over, but Cuba is sticking by the North Korean pariah.

This bond exposes Americans to grave risk. Analysts fret that Pyongyang is developing missiles and miniaturized warheads that will allow it to lob a bomb into the continental U.S. But having a desperate ideological pal 90 miles from U.S. shores magnifies the danger. In the past 21/2 years Cuba has tried to smuggle weapons to Pyongyang, engaged in high-level meetings with North Korean officials, and secured U.S. military technology. Anybody want to connect the dots?

On Friday, Wall Street Journal reporters Devlin Barrett and Gordon Lubold broke the story that the State Department became aware in June 2014 that a Hellfire missile had gone missing and that it was “likely in Cuba.”

Let’s face it: That was no shipping error, as some have speculated. Stealing weapons technology is what spies do for a living, and getting hold of a sophisticated piece of U.S. equipment is a major coup for Havana.

It is not a stretch to think that the regime will share, for a price, everything there is to know about the laser-guided, air-to-surface Hellfire—which can be launched from a helicopter or drone as well as from a plane—with its good friends Iran, Russia and North Korea, and even with other terrorist organizations.

President Obama seems to think that the Castros have abandoned their revolutionary obsession with harming the U.S. The theft of the Hellfire would have disabused even Chauncey Gardiner of such naiveté.

But not Mr. Obama. He was already engaged in a rapprochement with the regime when the State Department learned that Havana had the missile. If he issued an ultimatum that it be returned, his talks might have collapsed.

So six months later he went ahead with his plan to throw a lifeline to the economically struggling Castros by restoring diplomatic relations and liberalizing American travel to the island. In May Cuba was removed from the State Department’s list of state-sponsors of terrorism.

The missile is only the latest example of the no good that Cuba is still up to. In July 2013 Panama Canal authorities discovered 240 metric tons of weapons—including jet fighters and missiles—hidden under a sugar shipment aboard a North Korean ship that had sailed from Cuba and was bound for North Korea.

Havana tried to play down the incident, calling the weapons outmoded. But the U.N.’s North Korea sanctions committee said the shipment demonstrated “intent to evade UN sanctions” and that it was “consistent with previous attempts by” Pyongyang “to transfer arms and related materiel through similar tactics in contravention of Security Council prohibitions.” Samantha Power, the U.S. ambassador to the U.N., called it a “cynical, outrageous and illegal attempt” by the two countries to evade U.N. sanctions.

In the 13 months since Mr. Obama’s announcement that he would reopen a U.S. embassy in Cuba and use executive decrees to weaken the U.S. embargo, Cuba has repeatedly pledged its loyalty to North Korea. In March 2015, according to Cuba’s state-run news agency, North Korea’s foreign minister visited Havana and reminded Cubans that the two peoples “share a history of struggle together in the same trench against U.S. imperialism, which continues exerting economic pressure on our countries to this day.” The news agency also reported that the minister brought a “message from Jong-Un in order to expand and strengthen” the excellent relations between the two countries.

In June 2015 Raúl Castro hosted Kang Sok Su, the secretary of international relations for the North Korean Workers’ Party. In September Mr. Kim received Cuban Vice President Miguel Díaz-Canel in Pyongyang. Cuba’s state-owned newspaper Granma reported that Mr. Kim sent “an affectionate greeting” to the Castro boys during the visit. It also said that Messrs. Díaz-Canel and Kim discussed the two countries’ close relations and mutual cooperation.

This ought to worry U.S. national-security officials. But Mr. Obama is busy worrying about shaping his legacy. I’m not sure why: He’s the first U.S. president to bow to a Saudi king, the first to open the door for Iran to get the bomb, and the first to prop up the Castros even while they hold a stolen Hellfire missile. His place in history is already secure.